1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method and, more particularly, to a color image forming apparatus which forms images independently for respective color components, such as a laser beam printer (LBP), digital copying machine, or multifunction printer (MFP) using an electrophotographic process, and a color image forming method.
2. Description of the Related Art
As a kind of color image forming apparatus such as a printer or copying machine, there is known a tandem type color image forming apparatus which comprises electrophotographic image forming units equal in number to color components and sequentially transfers toner images of respective color components onto a print medium by the image forming units. The image forming unit of each color includes a developing unit and photosensitive drum. It is known that the tandem type color image forming apparatus has a plurality of factors which cause a positional error (to be referred to as a registration error) between images of respective color components.
These factors include the unevenness and attaching positional error of the lens of a deflecting scanning unit including the optical system of a polygon mirror, f θ lens, and the like, and the mounting positional error of the deflecting scanning unit to the image forming apparatus main body. Owing to these positional errors, the scan line does not become a straight line parallel to the rotating shaft of the photosensitive drum, and inclines or skews. If the degree of inclination or skew of the scan line (to be referred to as the profile or shape of the scan line hereinafter) is different between colors, a registration error occurs.
The profile has different characteristics for respective image forming apparatuses, i.e., printing engines, and for deflecting scanning units of respective colors. FIGS. 16A to 16D show examples of the profile. In FIGS. 16A to 16D, the abscissa axis represents a position in the main scanning direction in the image forming apparatus. Lines 160 to 163 expressed as straight lines in the main scanning direction represent the characteristics (profiles) of ideal scan lines free from a skew. Curves 164 to 167 represent the profiles of respective colors, and show examples of the profiles of scan lines for cyan (to be referred to as C hereafter), magenta (to be referred to as M hereafter), yellow (to be referred to as Y hereafter), and black (to be referred to as K hereafter), respectively. The ordinate axis represents a shift amount in the sub-scanning direction from an ideal characteristic. As is apparent from FIGS. 16A to 16D, the curve of the profile is different between colors. When electrostatic latent images are formed on the photosensitive drums of image forming units corresponding to the respective colors, the profile difference appears as the registration error between image data of the respective colors.
As a measure against the registration error, Japanese Patent Laid-Open No. 2002-116394 discloses a method of measuring the degree of skew of a scan line using an optical sensor in the process of assembling a deflecting scanning device, mechanically rotating the lens to adjust the skew of the scan line, and fixing the lens with an adhesive.
Japanese Patent Laid-Open No. 2003-241131 discloses a method of measuring the inclination of a scan line using an optical sensor in the process of mounting a deflecting scanning device into a color image forming apparatus main body, mechanically inclining the deflecting scanning device to adjust the inclination of the scan line, and then mounting the deflecting scanning device into the color image forming apparatus main body.
Japanese Patent Laid-Open No. 2004-170755 discloses a method of measuring the inclination and skew of a scan line using an optical sensor, correcting bitmap image data to cancel them, and forming the corrected image. That is, a shift of an actual scan line from an ideal scan line which is a straight line parallel to the surface of the photosensitive drum to the rotating shaft of the photosensitive drum is canceled by shifting image data by the same amount in an opposite direction. This method corrects image data, and thus does not require a mechanical adjustment member or adjustment step in assembly. This method can downsize a color image forming apparatus, and deal with a registration error at a lower cost than those by methods disclosed in Japanese Patent Laid-Open Nos. 2002-116394 and 2003-241131. The electrical registration error correction is divided into correction of one pixel and that of less than one pixel. In correction of one pixel, pixels are shifted (offset) one by one in the sub-scanning direction in accordance with the inclination and skew correction amounts, as shown in FIG. 17. In the following description, a position where the pixel is offset will be called a scan line changing point, and the process to offset a pixel will be called a scan line changing process. In FIG. 17, P1 to P5 are scan line changing points.
In FIG. 17, a profile 1701 of a scan line is corrected. The profile 1701 may also be expressed by an array of the coordinate values of pixels on a scan line, but in FIG. 17, is expressed by approximate straight lines divided for respective areas. The scan line changing point is a position in the main scanning direction where the profile is scanned in the main scanning direction and shifts by one pixel in the sub-scanning direction. In FIG. 17, P1 to P5 are scan line changing points. At a scan line changing point serving as a boundary, dots after the scan line changing point are shifted by one line in a direction opposite to the shift of the profile in the sub-scanning direction. This process is executed by paying attention to each line. An image 1702 represents an example of image data shifted in the sub-scanning direction at each scan line changing point. In FIG. 17, each hatched portion 1702a is one line before the scan line changing process, i.e., one line in original image data. As a result of the scan line changing process, each line shifts in a direction in which the shift of the profile in the sub-scanning direction is canceled. Image data 1703 represents an example of image data obtained in this manner. Each hatched portion is one line before correction. In image formation, corrected image data is formed for each line. For example, normal image formation proceeds in the order of a line 1721, line 1722, . . . . After image formation, a hatched portion which forms one line in image data before correction is formed on an ideal scan line which should be originally formed. However, the scan line changing process is done for each pixel, so a shift of less than one pixel still remains in the sub-scanning direction.
A shift of less than one pixel that cannot be completely corrected by the scan line changing process is corrected by adjusting the tone value of bitmap image data by preceding and succeeding pixels in the sub-scanning direction, as exemplified in FIGS. 18A to 18F. More specifically, when the characteristic represents an upward inclination, like a profile 1801 in FIG. 18A, bitmap image data before tone correction is corrected to a pixel array 1803 (shown in FIG. 18C) inclined in a direction (downward in this example) opposite to the inclination of the profile. FIG. 18B shows bitmap image data before correction. Image data 1802 is shifted by one pixel in the sub-scanning direction at scan line changing points P1 and P2, like a dot image 1806 in FIG. 18F. To make the image data 1802 close to the ideal image data 1803 after correction, tone correction is executed to smooth steps at the scan line changing points P1 and P2, like a bitmap image 1804 shown in FIG. 18D. FIG. 18D is a view schematically showing the densities of pixels by the width and intensity of a laser pulse for forming these pixels. After exposure, a latent image (exposure image) 1805 as shown in FIG. 18E is formed to smooth steps generated by the scan line changing process. According to this method, the image process can correct the registration error. Tone correction performed for smoothing after the scan line changing process will be called an interpolation process.
Conventional techniques have tried to cancel an unnatural step at a scan line changing point serving as a boundary by separately executing, in accordance with the profile characteristic of an image forming apparatus, correction in which the correction amount exceeds one pixel and correction in which the correction amount is less than one pixel. However, depending on the image forming method for image data for executing registration error correction, if correction of more than one pixel and that of less than one pixel are directly executed for image data, the image deteriorates. More specifically, when a continuous tone image is quantized by applying an error diffusion process, a quantization error generated when converting the pixel of interest into N-ary data is distributed to pixels (to be referred to as neighboring pixels around the pixel of interest hereinafter) around the pixel of interest that are processed after the pixel of interest. When the image forming unit forms an image based on image data having undergone the scan line changing process and interpolation process, the profile characteristic is canceled, and the image has a pixel arrangement close to one in original image data.
However, even if the image has a pixel arrangement close to one in original image data, the pixel arrangement is not completely the same, and a step of one line generated by the scan line changing process still remains. The interpolation process smooths this step, but only smooths the step by generating a density gradient. Hence, correction of more than one pixel at a scan line changing point and the error distribution do not match, and the mismatch appears as density unevenness or a texture, degrading the image quality.